Positioning and navigation systems can require an antenna that has high-accuracy and operates over multiple frequencies. FIG. 1 is a graph of operational frequencies for exemplary positioning and navigation systems. Modernized GPS System antennas and receivers operate along three bands, 1563 to 1578 MHz (L1), 1216 to 1240 MHz (L2), and 1164 to 1188 MHz (L3). A GPS system to be deployed is GALILEO. Although the exact bands of operation for GALILEO are unknown, it is anticipated that GALILEO will operate along five bands, 1165 to 1216 MHz (E5a and E5b), 1215 to 1237 MHz (E2), 1260 to 1300 MHz (E6), and 1563 to 1587 MHz (E1). Positioning and navigations systems can require frequency operation between 1.15 and 1.60 GHz band.
Positioning and navigation systems can also require elimination of multipath signal reflections. FIG. 2 is a diagram of an example of a positioning system. The positioning system includes two transmitting GPS systems 202a, 202b and one GPS receiver 204. Multipath signal reflections 206a, 206b, 206c, 206d, generally 206, are reflected from ground 208, a building 210, a tree 212 and an antenna 214. The multipath signal reflections 206 interfere with an information signal 216a, 216b (i.e., the primary direct line-of-sight signal) from the two transmitting GPS systems 202a, 202b. The multipath signal reflections 206 reduce accuracy of the position data.
Axial-ratio is one measure of multipath signal rejection capability for Right Hand Cross Polarized (RHCP) antennas, such as GPS antennas. Multipath signals are primarily Left Hand Cross Polarized (LHCP) reflection signals from objects located within a close proximity to the antenna. Current high accuracy GPS antennas feature an axial-ratio bandwidth that is too narrow to cover frequencies between 1.15 and 1.60 GHz
Current antenna systems can eliminate multipath signal reflections and achieve sufficient antenna performance for two bands, L1 and L2, using for example, GPS antennas equipped with choke ring ground planes. The choke ring ground plane efficiently mitigates multipath signal reflections at L1 and L2 by eliminating propagation of surface wave on the ground plane and thereby suppressing undesired multipath signals at low elevation angles. The choke ring ground plane enhances antenna performance by reducing back lobe and side lobe radiation that also improves multipath signal reflection mitigation.
Plane waves and surface waves that travel on a finite sized non-corrugated metal ground plane radiate causing ground plane edge diffraction, thus increasing back lobe and side lobe radiation. A choke ring is a corrugated surface having deep metal concentric rings. Corrugated surfaces do not support propagation of plane waves. Consequently, choke rings to do not support propagation of plane waves. Moreover, for a choke ring to ensure the absence of propagation of surface waves the corrugation depth (i.e. concentric rings) d, must be λ/4≦d≦λ/2, for each frequency of operation (operation at cutoff). The absence of propagation of surface waves eliminates the antenna back lobes and side lobes, thus preventing reception of multipath signals at low elevation angles. For a dual-frequency, L1 and L2, GPS antenna to operate with surface wave cutoff, the corrugation depth is typically between 61 mm≦d≦95 mm and the diameter of the choke ring is typically approximately 360 mm.
Current high accuracy antennas cannot support frequencies over the entire range of 1.15 to 1.60 GHz.